User can view a trip

https://trello.com/c/cIYUtiPe/36-as-a-user-i-can-show-a-trip

![show a trip](https://cloud.githubusercontent.com/assets/3759510/5076448/ecf331c2-6e68-11e4-8a7e-26f1610c4822.gif)

I think Trip.find works, while to my understanding current_user.trips would be faster as it would narrow down the data to be searched.